Future-generation multimedia wireless communication systems, which are a recent active research area, require processing of various types of information including video data and wireless data at high data rates, beyond the traditional voice-oriented service.
Owing to its advantage of high data rate, Orthogonal Frequency Division Multiplexing (OFDM) has recently attracted much attention. OFDM is a special case of Multi-Carrier Modulation (MCM) in which a frequency band is divided into multiple orthogonal subcarriers and data is transmitted on the subcarriers. The low complexity of OFDM reduces inter-symbol interference. In OFDM, an input serial sequence of data symbols is converted to N parallel data symbols and transmitted on N separate subcarriers.
Orthogonality is maintained among the subcarriers in the frequency domain. Each orthogonal channel experiences mutually independent frequency-selective fading and the resulting increase in the gap between transmitted symbols leads to minimum intersymbol interference. Orthogonal Frequency Division Multiple Access (OFDMA) is a multiple access scheme in which multiple accesses are allowed through independent allocation of part of available subcarriers to each user in an OFDM system. In OFDMA, it is typical to independently allocate frequency resources, namely sub-carriers to a plurality of users in such a manner that the frequency resources are not overlapped with one another. As a result, allocation of frequency resources is mutually exclusive among users in OFDMA. The OFDMA system may achieve frequency diversity for multiple users by frequency selective scheduling and allocate subcarriers in various manners according to subcarrier permutation.
In the mean time, a base station (BS) can simultaneously transmit multiple layers for one or several users. To this end, the multiple data layers can be multiplexed into a certain frequency/time resource and transmitted to the one or several users on the certain frequency/time resource. In general, a maximum available transmit power is predetermined for downlink transmission of a BS according to a bandwidth supported by the BS, the throughput of the BS, the power efficiency of the BS, etc. It is required that the BS efficiently allocates transmit power per subcarrier in an OFDM symbol since a total transmit power available for the BS at a time is usually limited to a certain value. Besides, a UE should know the relationship between transmit power for data and transmit power for a reference signal in order to demodulate the data received from the BS.
Accordingly, there exists a need for a method for allocating transmit power efficiently in a BS, while simplifying the process of determining downlink transmit power or a power ratio between a data signal and a reference signal at a UE.